1. Field of the invention:
This invention relates to an index guided semiconductor laser device which attains laser oscillation at an extremely low threshold current level without creating astigmatism.
2. Description of the prior art:
Conventional semiconductor laser devices are classified into two groups, gain-guided semiconductor laser devices and index guided semiconductor laser devices, according to their optical waveguiding mechanism. Index guided semiconductor laser devices are superior to gain-guided semiconductor laser devices in view of transverse mode stabilization which is important in practical use. Index guided semiconductor laser devices having a variety of structures have been proposed, typical examples of which are BH (buried heterostructure) lasers and VSIS (V-channeled substrate inner stripe) lasers.
FIG. 2(A) shows a BH laser device, in which a multi-layered crystal structure having the width W is formed into a stripe on the p-GaAs substrate 1 so as to function as a laser oscillation operating area. The multi-layered crystal structure constitutes a double-heterostructure which is composed of a p-GaAlAs cladding layer 2, a GaAlAs active layer 3, an n-GaAlAs cladding layer 4, and an n-GaAs cap layer 5. A substance 15 having a low refractive index is buried outside of the active layer 3, resulting in an index guided structure creating no astigmatism and having a threshold current of as small as about 10 mA. However, if a proper refractive index is not applied to the burying layer 15 and if a proper width is not applied to the waveguide, the device will oscillate in a high-order transverse mode. Thus, the BH laser device is disadvantageous in that it is restricted by production conditions.
FIG. 2(B) shows a VSIS laser device, which is produced as follows: On a p-GaAs substrate 1, an n-GaAs current blocking layer 6 is formed by liquid phase epitaxy. Then, a striped V-channel having the width W is formed in the substrate 1 through the current blocking layer 6, resulting in a current path. Then, on the current blocking layer 6 including the V-channel, a p-GaAlAs cladding layer 2, a GaAlAs active layer 3, an n-GaAlAs cladding layer 4, and an n-GaAs cap layer 5 are successively formed by liquid phase epitaxy, resulting in a double-heterostructure for laser oscillation operation. Even when the width W of the waveguide is set at a value of as large as 4-7 .mu.m, since laser light outside of the waveguide within the active layer 3 is absorbed by the substrate 1, highorder mode gain is suppressed and a high-order transverse mode does not occur. However, the threshold current of this VSIS laser device is 40-60 mA, which is extremely higher than that of the BH laser device. Moreover, the astigmatism arising from this VSIS laser device is as large as 10-20 .mu.m.
The reason why the threshold current of the VSIS laser device becomes high is as follows: Current injected into the device is confined within the inner striped structure formed by the current blocking layer 6, but carrier injected into the active layer 3 diffuses into the outside of the active layer 3, resulting in carrier unusable for laser oscillation. FIG. 3 shows the distribution of carrier density n in the junction direction y within the active layer of the VSIS laser device, indicating that when the waveguide width W is 4 .mu.m, carrier at the shaded areas (corresponding to the outside of the waveguide) is unusable for laser oscillation. The unusable carrier results in unnecessary light and/or generating unnecessary heat, causing a decrease in reliability of the device. Moreover, the above-mentioned large astigmatism of the VSIS laser device arises because light outside of the waveguide is absorbed by the substrate and the wave front of said outside light is propagated later than that of light inside of the waveguide.